1. Field of the Invention
The invention relates to a method and apparatus for determining the remaining operating time of an appliance powered by batteries or accumulators.
2. Description of the Related Art
Generally this remaining operating time is determined in that the charge condition is determined from the battery discharge curve by ascertaining the voltage state and, based on the voltage state, the remaining operating time is determined and ultimately displayed.
However, this method according to the state of the art has various defects and problems:
After switching off the appliance and storing it in the switched-off condition, the battery voltage recovers in such a way that, when the appliance is switched on again, an excessively long remaining operating time is indicated. The error associated therewith can amount to a multiple of the remaining operating time actually available, particularly towards the end of battery charging and after relatively long storage in the unused condition so that the resting voltage curve is obtained.
Another source of error is the fact that known display devices have set predetermined battery discharge curves and apply this discharge curve even if accumulators (secondary cells) and not batteries (primary cells) are used, whereby an intrinsic error occurs which indicates a clearly false remaining operating time.
The main reason for this is that the remaining operating time forecast in the case of accumulators encounters various problems which arise from the characteristics of the accumulators themselves (past history within the charge cycles, characteristics of the preceding charge cycles, holding or not holding deep discharges, memory effect or disturbances caused by temporary short-circuiting of the accumulator, etc.). Therefore, as indicated above, battery charge curves are generally presupposed and the errors in the case of accumulators are accepted.
Determining apparatus of this type are used in various appliances, in particular battery-powered microphones, headphones, tape recorders, mobile telephones and the like.
The invention aims to devise an apparatus in which the aforementioned problems are at least substantially diminished or entirely obviated.
According to the invention, this is achieved in that an EEPROM is provided, which stores the value of the battery voltage at the instant of switching off the appliance. This measure makes it is possible to obviate the error which arises as result of the voltage recovery of the battery during the off-time so that, particularly when the appliance is switched on, the major error, depending on its value, occurring in the state of the art is obviated. Since it is also often decided when switching on the appliance whether new batteries are to be installed or at least held in readiness, with the display according to the invention this decision can be made with a substantially higher degree of reliability.
In one development the apparatus is provided with a battery change detector which detects changing of the batteries. For example, this may be a contact which monitors the opening of the battery compartment, or a control circuit which detects whether the contact between the appliance and the battery has been interrupted, similar to the theft protection in automobile radios.
This measure makes it is possible, in such a case, for example to assume that the newly installed batteries have been stored for a relatively long time without load, for which reason the voltage value obtained upon switching on is taken as a no-load value and the indicated remaining operating time is corrected accordingly.
Finally, in a preferred development it is provided that when switching on the appliance or when the battery charge detector has recognised the replacement, a load test is performed for the installed batteries so that it is possible to distinguish whether batteries or accumulators have been installed.